Phase-locked loops (PLLs) are typically used to provide frequency selectivity for demodulation of frequency modulated (FM) signals in numerous communications equipment such as satellite communications equipment, stereo receivers, FM radios, stereos, and modems. A PLL is characterized by its ability to track a varying input frequency signal and as a signal conditioner. Since the PLL locks onto only the desired frequency component of the input signal, typically a voltage-controlled oscillator in the PLL reproduces the input signal at the same frequency, but without noise and interference.
A good phase-locked loop should have a fast tracking speed to track possible signal phase change and a small noise bandwidth to reduce output phase jitter. However, the fast tracking speed and small noise bandwidth requirements are conflicting. That is, for a given set of phase-lock loop parameters, a fast tracking speed will mean a larger noise bandwidth, while a smaller noise bandwidth will necessarily reduce tracking ability.
Traditionally, in high speed modems, gear-shifting is used to change PLL parameters at different stages of operation. For example, in a training stage, a set of parameters corresponding to fast tracking is used to ensure proper tracking, and in steady state operation, another set of parameters corresponding to a selected small noise bandwidth is used to reduce phase jitter. Frequently, multiple gear-shifting stages are required to ensure proper operation.
An inherent disadvantage of the above approach is that the PLL parameters have to be set to accommodate a worst case tracking requirement specified in a modem design, unduly increasing the output phase jitter. While the worst case seldom occurs, the increased noise bandwidth is always being experienced. A second disadvantage is the PLL's poor ability to adapt to changing operating conditions, such as sudden timing source or carrier phase changes. To be able to recover from a sudden change, the PLL has to operate in a mode faster than necessary for normal operation, again unduly increasing phase jitter.
As the data rate for modems has increased, toleration of phase jitter has decreased. The phase jitter increase due to the worst case tracking requirement in conventional PLLs has created a data transmission speed limitation for modems. There is a need for a PLL that can self-adjust, providing tracking ability when and where needed, while maintaining minimal possible noise bandwidth.